1. Field of the Invention
The present invention relates to an inkjet recording head and an image forming apparatus comprising an inkjet recording head, and in particular, to maintenance technology for an inkjet recording apparatus which prevents the viscosity of liquid at a meniscus surface from increasing and enables stable ejection.
2. Description of the Related Art
As an image forming apparatus in the related art, an inkjet printer (inkjet recording apparatus) is known, which comprises an ink ejection head (inkjet recording head) having an arrangement of a plurality of nozzles and forms images on a recording medium by ejecting ink liquid droplets from the nozzles in the ink ejection head toward the recording medium while relatively moving the ink ejection head and the recording medium to each other.
Various methods are known in the related art as ink ejection methods for an inkjet recording apparatus of this kind. For example, a piezoelectric method, a thermal inkjet method, and the like, are known. According to the piezoelectric method, the volume of a pressure chamber (ink chamber) is changed by deforming a vibration plate which forms a portion of the pressure chamber because of deformation of a piezoelectric element (piezoelectric actuator), thereby supplying ink into the pressure chamber from an ink supply channel when the volume is increased and ejecting the ink inside the pressure chamber from a nozzle when the volume of the pressure chamber is reduced. Moreover, according to the thermal inkjet method, ink is heated to generate a bubble, thereby ejecting the ink on the basis of the energy of the bubble expansion.
In such an image forming apparatus including an ink ejection head as an inkjet recording apparatus, ink is supplied to the ink ejection head from an ink tank which stores the ink, via the ink supply channel, and the ink can be ejected by any one of the various ejection methods described above. It is preferable that the ink used in such an image forming apparatus be rapidly dried (evaporated) and fixed immediately after the ink droplets deposit onto the recording medium.
On the other hand, the nozzles in the ejection head are filled with ink at all times so as to carry out printing immediately whenever a print instruction is issued. If the ink inside the nozzles dries, then the ink ejection from the nozzles becomes unstable and therefore, during non-printing, the ink ejection head is sealed tightly with a cap so as to prevent the ink in the nozzles from drying.
Moreover, in a case of an image forming apparatus of a shuttle scanning type in which an ink ejection head moves back and forth reciprocally over the paper, during printing, ejection failures are prevented by expelling ink having an increased viscosity by means of a method where the piezoelectric elements are driven to perform ink ejection or the nozzles are suctioned by means of a negative pressure when the ink ejection head is in a position outside the paper. On the other hand, in an image forming apparatus of a line type ink ejection head which corresponds to the paper width and is suitable for high-speed printing, it is difficult to perform ejection and suctioning of this kind during printing.
In the image forming apparatus using a line type ink ejection head corresponding to the width of the paper, the ink in the nozzles is exposed to the air especially during printing, and the ink in the nozzles which have not performed ejection for a long period of time dries. Therefore, the viscosity of the ink rises and the ink at the meniscus surface increases. Consequently, there is a possibility that the nozzles are subjected to ejection failures due to the nozzle blockages or disappearance of the ink in the nozzles.
In order to prevent the viscosity of ink at a meniscus surface from increasing, Japanese Patent Application Publication No. 2003-191470 discloses an inkjet recording head that includes an orifice plate (nozzle plate) which has nozzles and is made of a porous member which can be impregnated with ink. In this inkjet recording head, a moisturizing liquid or ink is supplied to the porous member, thereby moistening the periphery of the meniscus and preventing the viscosity of ink at the meniscus surface from increasing.
However, this technology in the related art involves following problems.
In the technology disclosed in Japanese Patent Application Publication No. 2003-191470, the inkjet recording head has a composition in which an ink-repelling film is formed on the ejection side surface of the orifice plate (nozzle plate) made of the porous member in order to prevent the ink from leaking through the orifice plate to areas other than the periphery of the nozzles. In this case, the position at which the meniscus surface makes contact with a nozzle (the inner wall of nozzle) corresponds to an interface between the orifice plate made of porous material and the ink-repelling film. Accordingly, the following problems with the ink ejection may be created.
Specifically, in an inkjet recording head that includes piezoelectric actuators and adopts a pull-push ejection method which is one of the general ejection control methods and in which the meniscus is first pulled inward in a direction opposite to the ejection direction and then the ink is pushed outward, the meniscus infiltrates into the porous orifice plate. Hence the meniscus surface shape becomes asymmetrical in accordance with the shape of the porous material, and hence the ink ejection direction may be deflected or ejection failures may occur because air bubbles enter the porous member.
Moreover, even if a push ejection method in which the ink is ejected by pushing without first pulling the meniscus is used in order to avoid such problems, residual vibration (resonance) of the meniscus after ejection, which also occurs in the pull-push ejection method described above, occurs. Since ejection has already finished, then the residual vibration does not affect the ejection direction. However, in this case, air bubbles may be incorporated into the porous member, and thereby ink may not be ejected in the subsequent ejection operation. In the related art, although drive waveforms have accordingly been designed so as to reduce residual vibration of this kind, it is difficult to eliminate the influence of residual vibration completely.
Furthermore, in the technology disclosed in Japanese Patent Application Publication No. 2003-191470, if there is an increase in the viscosity of the ink at the meniscus surface, then suctioning is carried out by applying a negative pressure to the orifice plate made of a porous member, prior to printing a new sheet or page, thereby removing the ink of increased viscosity. However, the above technology is not effective against the ejection defects occurring as a result of the viscosity increase during printing a single sheet or page. Moreover, according to the above technology, the print speed declines because suctioning is carried out (if necessary) prior to printing a new sheet or page.